Fiber optics refers to the technical art of transmitting light from place to place over a thin strand of glass known as an optical fiber. Optical fibers can carry light over great distances with little loss. Optical fibers are also flexible and allow light to be directed around corners, for example, without the use of mirrors.
A most important application of fiber optics is in telecommunications where voice signals or internet data may be carried by light traveling through optical fibers. An optical fiber is capable of carrying much more information than a copper wire. The rise of the internet especially has driven demand for more communications capacity and more fiber optic systems.
Fiber optic systems include many types of components to perform functions such as converting data into light signals, amplifying or attenuating light signals, and combining several signals on one fiber. Optical attenuators are devices that reduce the power of light in a fiber. The amount of attenuation in a variable optical attenuator is easily adjustable, for example by electronic control.
Optical attenuators are used for power management, equalization among different wavelength channels, gain control in amplifiers, overload protection and other tasks. It is convenient to include variable optical attenuators in a fiber optic system in order to easily adjust light power in various parts of the system.
Modern fiber optic systems carry several different colors or wavelengths of light simultaneously. Each wavelength channel may be dedicated to its own stream of optical data. This method, known as wavelength division multiplexing, allows more data to be carried on a single fiber than if only one wavelength is used.
When an optical attenuator is used in a fiber optic system that operates with several wavelengths simultaneously it is desirable for the amount of attenuation to be the same for each wavelength.
It has become quite popular to use micro-electro-mechanical systems (MEMS) in conjunction with fiber optics to build variable optical attenuators (VOAs). Many MEMS VOAs are based on tilting mirror technology. An input fiber and an output fiber are arranged so that a tiny mirror reflects light from one to the other. When the mirror is tilted the amount of light that is transferred between the fibers is reduced because the reflected light partially misses the output fiber.
Tilting mirror MEMS VOAs have been quite successful; their technical characteristics are not ideal, however. A key problem with conventional MEMS VOAs is that as the amount of attenuation is increased it becomes unequal for different wavelengths. The attenuation depends on wavelength.
Wavelength dependent loss (WDL) is a measure of the difference in attenuation at different wavelengths for an optical attenuator. As an example, consider an optical attenuator that works at wavelengths in the range between 1.53 μm and 1.57 μm and is operating at an average attenuation of 10 dB. If the actual attenuation is 9.75 dB at 1.53 μm and increases to 10.25 dB at 1.57 μm, then the WDL over the range 1.53 μm to 1.57 μm is 0.5 dB (10.25−9.75=0.5) when the average attenuation is 10 dB.
In conventional VOAs the WDL increases as the level of attenuation increases. In other words, continuing the example above, the difference in attenuation at 1.53 μm versus 1.57 μm becomes greater as the overall or average attenuation increases. As the overall attenuation increases to 20 dB, for example, the WDL might increase to 1.0 dB.
It would be desirable to have a tilting mirror MEMS VOA with as small WDL as possible over a wide range of operating wavelengths and attenuation levels. Even if it not possible to eliminate WDL at all attenuations, it would still be very useful to reduce WDL wherever possible. For example, it would be desirable to minimize WDL at 5, 10, 15 dB or any other attenuation at the option of the attenuator designer.